apt not upgrading package

[Colin Law]

I have added a ppa for mosquitto, when I run
sudo apt-get update && sudo apt-get dist-upgrade
it says:
0 to upgrade, 0 to newly install, 0 to remove and 0 not to upgrade.
but apt-cache policy for mosquitto shows:
$ apt-cache policy mosquitto
mosquitto:
  Installed: 1.4.8-1build1
  Candidate: 1.4.9-0mosquitto1
  Version table:
     1.4.9-0mosquitto1 500
        500 http://ppa.launchpad.net/mosquitto-dev/mosquitto-ppa/ubuntu
xenial/main amd64 Packages
 *** 1.4.8-1build1 500
        500 http://gb.archive.ubuntu.com/ubuntu xenial/universe amd64 Packages
        100 /var/lib/dpkg/status

Which says there is a candidate for update.  I cannot see why apt-get
does not offer the upgrade.  If mosquitto were held then I think
apt-get would show it as held back.  Any suggestions as to what is
going on?

Colin